Field of the Invention
The present disclosure relates to a thin film type controlled viewing window back light unit and a thin flat type Controlled Viewing window Display (or ‘CVD’) using the same. Especially, the present disclosure relates to a thin film type controlled viewing window back light unit adopting the holography technology and a thin flat type controlled viewing window display using the same.
Discussion of the Related Art
Recently, many technologies and researches for making and reproducing the 3D (Three Dimensional) image/video are actively developed. As the media relating to the 3D image/video is a new concept media for virtual reality, it can improve the visual information better, and it will lead the next generation display devices. The conventional 2D image system merely suggests the image and video data projected to a plan view, but the 3D image system can provide the full real image data to the viewer. So, the 3D image/video technologies are the True North image/video technologies.
Typically there are three methods for reproducing 3D image/video: the stereoscopy method, the holography method and the integral imaging method. Among them, the holography method uses laser beams so that it is possible to observe the 3D image/video with naked eyes. The holography method is the most ideal method because it has an excellent visual stereoscopic property without any fatigue of observer.
To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the light from the scene or object (the object beam). If these two beams are coherent, optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film, which is called the hologram. The central goal of holography is that when the recorded grating is later illuminated by a substitute reference beam, the original object beam is reconstructed (or reproduced), producing a 3D image/video.
When a display system is implemented using a holography technology according to the related art, it is very hard to get evenly distributed brightness because the intensity of the light radiated from the light source follows the Gaussian Profile. In addition, when the incident light form the light source has the inclined incident angle in order to reduce the high order diffraction components causing the image noise, the collimation degree of the laser may be lowered.
In order to solve or address the drawbacks of the related art, there are some researches for providing the back light unit BLU which can maintain the collimation degrees of the incident light even though the incident light has the inclined angle for reducing the high order diffraction components. For example, the system using the collimation lens was presented. FIG. 1A is a figure illustrating a schematic structure of the back light unit BLU suggesting a collimated light beam using the collimation lens, according to a related art.
Referring to FIG. 1A, disposing a point light source 30 at the position of the light source and positioning a collimation lens CL at the focal length position apart from the light source 30, the lights radiated from the point light source 30 may be formed as the collimated beam light by the collimation lens CL. This collimated light beam can be used as a reference light beam in the non-glasses type display system.
However, in the most cases of the holographic display system, it is preferred that the reference light beam is incident into the diffraction optical element with an inclined angle from the vertical direction to the incident surface of the element. The reason is that, as the diffraction element like the holographic film may generate the 0th mode image and/or 1st mode image and they may work as noises in the holographic image, the 0th mode and/or 1st mode should be reduced or eliminated. To do so, it is easy way for reducing or eliminating these noises to make an incident angle to the incident reference light beam.
For example, the position of the point light source 30 may be shifted at any one side to make the incident angle in the back light unit shown in FIG. 1A. FIG. 1B is a figure illustrating a schematic structure of the back light unit BLU generating a collimated light beam using the collimation lens in which the collimated light beam has an incident angle, according to the related art.
Referring to FIG. 1B, the point light source 30 may be shifted or moved to upside from the light axis 130 so that the incident angle from the light axis forwarding to the center of the lens CL may be α. Then, theoretically, as the dotted line in FIG. 1B, the collimated light beam has the incident angle α from the light axis 130. However, in actual case, by the physical characteristics such as the spherical aberration, the real light path may not be collimated and/or paralleled with the incident angle α, as the solid line in FIG. 1B. As a result, the light beam from the back light unit BLU may not be incident into the wanted area and/or wanted direction evenly but be unevenly distributed over the incident surface of the diffraction element.
As one method for solving this problem, by combining the prism sheet with the collimation lens, the back light unit of which the light direction can be controlled is suggested. Hereinafter, referring to FIG. 2, we will briefly explain about the light direction controllable back light unit. FIG. 2 is a figure illustrating a schematic structure of the back light unit suggesting a collimated light beam which direction can be controllable according to the related art.
The light direction controllable back light unit BLU according to the related art comprises a collimated lens CL, a point light source 30 disposed one side of the collimation lens CL and a prism sheet PS disposed at the other side of the collimated lens CL. The point light source 30 may be any type of light source which can radiate lights to radial directions from one point. In order to radiate most of all lights from the point light source 30 to the collimation lens CL, a mirror (not shown) may be further included at the back side of the point light source 30.
The point light source 30 can be preferably disposed at the focal plane of the collimation lens CL. Especially, the point light source 30 can be more preferably positioned on the light axis 130 connecting between the center point of the collimation lens CL and the center point of the forcal plane of the collimation lens CL.
The collimation lens CL may change the lights radiated from the point light source 30 into a collimated light beam 100. That is, the collimated light beam 100 may radiate to one direction parallel to the light axis 130. The collimation lens CL may include any one of the optical lenses such as the fresenl lens.
It is preferred that the prism sheet PS is positioned as being opposite the point light source 30 across the collimation lens CL. The prism sheet PS may refract the light direction with certain angle α as being inclined to the light axis 130. By the prism sheet PS, the parallel property of the collimated light beam 100 may be maintained and the propagation direction of the collimated light may be redirected to downward with angle of a from the light axis 130. As a result, the prism sheet PS can change the collimated light beam 100 into the controlled collimated light beam 200. The prism sheet PS may include a Fresnel prism sheet.
The holographic display system using these back light units can be applied to the hologram 3D display or the controlled viewing-window display and so on. Particularly, the controlled viewing-window display can be applied to the various display systems.
For one example, as the viewing window can be controlled, it can be applied to the security display system in which the display information is presented to the specific persons only. For another example, it can be applied to the multi-viewing display system in which different video data can be provided to different positions (or ‘viewing areas’). Further, as the left eye image and the right eye image can be provided to the left eye and the right eye, respectively without any interference, a good 3D display can be designed.
FIG. 3 is a figure illustrating the schematic structure of the controlled viewing-window display according to the related art. Referring to FIG. 3, the controlled viewing-window display according to the related art comprises a display panel LCP representing video data and a back light unit BLU. The display panel LCP may be a flat panel display using a back light system, such as the liquid crystal display panel. The controlled viewing-window display suggests the display information represented on the display panel LCP into the certain area (or ‘the specific viewing window’). In order to control the viewing window, the back light unit BLU in which the radiating area of the back light can be controlled is required. For example, the back light unit BLU may be a system adopting light control system as shown in FIG. 2.
In detail example, the back light unit BLU for the controlled viewing-window display according to the related art may include a light source LED, a lens LEN, a reflection plate REF and a holographic film HOE. In order for applying the holographic technology, it is preferable to use the highly collimated light beam. Therefore, it is preferable that the light source LED may be a laser or a light emitting diode laser. For the case that the light source LED is the general light emitting diode, a collimation lens LEN may further included in order to get the collimated light beam. The holographic film HOE is for making the back light radiating to a specific viewing area using the collimated light. By radiating the back light as a reference light beam to the holographic film HOE, the back light of which radiating area can be controlled according to the recording pattern of the holographic film HOE may be provided to the display panel LCP.
In order to develop the large area controlled viewing-window display, a large area holographic film HOE corresponding to the large area of the display panel LDP should be disposed at the back side of the large area display panel LDP. Further, a reflection plate REF may be included to send the back light radiated from the light source LED and collimated by the collimation lens LEN to the large area holographic film HOE.
As mentioned above, the controlled viewing-window display should include the lens LEN and the reflection plate REF for optically converging and diversing the light. Therefore, in order to provide the highly collimated light, there should be a physical space for ensuring enough light paths. That is, the back light unit BLU should require a large volume space. However, as the controlled viewing-window display according to the related art would have volumatic space and heavy weight, it is hard to apply the backlight unit of the related art to various display systems.